requirements: Cap pyrsistent on Python 2

I can't reproduce this locally, but it's breaking Travis.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

requirements-test.txt

@@ -2,4 +2,5 @@ mysqlclient~=1.4.4  # pyup: >=1.4.4,<1.5.0
 psycopg2-binary~=2.8.0  # pyup: >=2.8.0,<2.9.0
 sqlparse~=0.3.0  # pyup: >=0.3.0,<0.4.0
 python-dateutil~=2.8.0  # pyup: >=2.8.0,<2.9.0
+pyrsistent<0.16.0; python_version < '3.0'  # pyup: ignore
 openapi-core~=0.8.0  # pyup: >=0.8.0,<0.9.0